Mapping ground elevations for a series of locations in an area, such as for creating a topographical map of the area, may be accomplished by traversing the area and periodically measuring positional and elevation data for various locations in the area. Measuring the position and elevation of each location is typically accomplished using a global positioning system (GPS) receiver, such as a wide area differentially corrected global positioning system (WADGPS) receiver, or the like. When creating a topographical map of an area for slope sensitive applications, such as for analyzing water flow, drainage for agricultural fields, and the like, relative elevation accuracy is required. For example, topographical maps for analyzing water flow and/or drainage for agricultural fields may require relative accuracies in the 1 to 2 centimeter range. It should be noted that absolute vertical accuracy is not required for such applications, as long as the elevation measurements are taken relative to a consistent baseline.
Real-Time Kinematic (RTK) systems may be used to provide elevation measurements for producing survey quality topographical maps. These maps aid in the analysis of water flow and drainage for agricultural fields, as well as for other slope sensitive applications outside agriculture. These applications typically require elevation measurements having accuracies in the 1 to 2 centimeter range relative to a consistent baseline. Because of atmospheric interference with satellite signals received by a GPS receiver, such as interference from the ionosphere, troposphere, cloud cover, weather, and the like, elevation data measured by a GPS receiver may tend to drift over time. RTK systems continuously compare data received from a GPS system to positional and elevation measurements for a surveyed location, such as a surveyed base station. The data comparison is used to generate correction factors for adjusting measurements taken by GPS receivers in the vicinity of the surveyed base station, since the satellite signals received by the GPS receivers will be subject to atmospheric interference similar to that detected by the RTK system at the base station.
RTK systems use radio links with the base station to send the correction factors to GPS receivers for adjusting elevation measurements taken by the receivers, subsequently allowing corrected measurements to have absolute vertical accuracies in the range of several centimeters. The absolute vertical accuracies of the corrected elevation measurements ensure relative accuracies between the measurements. The range of the equipment used to transmit and receive signals over the radio links, as well as differing atmospheric conditions in the vicinity of the base station, which may make correction factors less accurate than desired, limit the area over which an RTK system may provide the accuracy desired to within a relatively small radius of a surveyed base station. Typically, this radius is a maximum of 10 to 20 kilometers. Additional problems with RTK systems include the need for achieving line of sight between radio transmitters and receivers, as well as the presence of external interference sources which may use the same band as a radio link, interfering with the reception of signals from an RTK system.
Consequently, it would be advantageous to provide a method for acquiring elevation data for an area using a GPS receiver, such as a wide area differentially corrected GPS receiver, or the like, without requiring communication with a base station of an RTK system or the like.